If you have a job and operate a sideline business, using your home office for both your employer's business and your own will disqualify you from taking the home office deduction. Remember, your home office must be used regularly and exclusively for your own business.
If you sell merchandise and use separately identifiable space in your home (or garage) for regularly storing inventory, you may allocate a portion of your home's indirect expenses to that space. For example, if the house is 2,000 S.F. and the storage space is 200 S.F., you can allocate 10% (200/2,000) of the total indirect home expenses to the storage space.
A desk, telephone with a separate line, files, computer, printer, and fax, are evidence that you are using the area for business. If you use your den for office space, a partition would help to separate the business area, but it is not required.